The present invention relates to a biaxial oriented polyester film which is linearly tearable in a longitudinal direction and/or a width direction of the film, has excellent strength, has excellent heat resistance and has excellent dimensional stability. The film is useful as a packaging material for foods (e.g. pickle, soybean paste, soup, jam, etc.), frozen foods, refrigerated foods, retort-packed foods, drugs, daily necessities and toiletries.
Various packaging bags made of plastic films are exclusively used for packaging foods, drugs, miscellaneous goods, etc., and packaging bags produced by laminating two or more biaxially oriented plastic films and heat-sealable non-oriented plastic film are widely used. In these fields excellent durability, moisture resistance, mechanical strength, heat resistance and oil resistance are required, particularly, high-strength biaxially oriented polyester films produced by a flat simultaneous biaxial orienting method, a roll and tenter serial biaxial orienting method, etc. are used as the biaxial oriented film.
The packaging bags produced from these biaxially oriented polyester films have a problem that the tearing property is poor. In order to improve the tearing property, a notch is exclusively provided. However, sometimes linear tearing of the packaging bag in the longitudinal or lateral direction can not be conducted, in case of tearing from this notch. In that case, not only the contents scatter on opening resulting in waste, but also an accident such as contamination of clothes is liable to arise when the contents are liquid. In case of the biaxially oriented polyester film, particularly, these problems often arise. Therefore, a method of laminating heat-sealable non-oriented plastic films to reduce the seal strength, producing a bag and opening the bag from the sealed portion has been used. However, an improvement is needed because of a phenomenon that the bag can not be opened and that the contents scatter from the sealed portion due to handling before opening during the transportation.
Examples of the easily openable material, which are superior in linearly tearable property when tearing the film, include those produced by using a uniaxially oriented polyolefin film as an intermediate layer in case of laminating. For example, there is a three-layer laminated film of a biaxial oriented polyester film/uniaxially oriented polyolefin film/non-oriented polyolefin film. In this case, although the linearly tearable property of the uniaxial oriented film in the orienting direction is improved, the laminated film has a multi-layer structure wherein a new intermediate layer is provided only for improving the linearly tearable property, thereby causing a problem of high cost. Therefore, use thereof was limited.
Japanese Patent Kokoku Publication No. 12871/1980 discloses a biaxial oriented film obtained from a mixture of a polyester comprising a dibasic acid and a glycol, and a block copolymer polyester comprising a polyalkylene glycol as a diol component. This film is characterized by its excellent adhesion to printing ink, but does not disclose that the tearing property is improved by setting a mixing ratio of the polyester to the block copolymer polyester to a specific ratio and setting an illuminance anisotropic ratio (IAR) of transmitted scattering light of this mixture within the range from 0.1 to 1.0 at all.